The present invention relates to an air flow sensor using a heated resistor (hereafter referred to as a heated resistor type air flow sensor), which measures the flow rate of air with a heated resistor, and especially to a heated resistor type air flow sensor suitable for measuring the flow rate of air taken into an internal combustion engine.
A heated resistor type air flow sensor has mainly been used for an air flow sensor, which is located in the path of intake air in an internal combustion engine such as a car, to measure the amount of the intake air, because this type air flow sensor can measure the mass flow rate of the intake air. Further, a heated resistor type air flow sensor in which a thin-film sensor region is formed on a semiconductor (for example, silicone) substrate by using a fine processing technique for semiconductors, has attracted special interest recently, because such an air flow sensor using a semiconductor can easily be fabricated by a mass-production method, which in turn reduces the fabrication cost of the air flow sensors, and it can also be driven with low power. An air flow sensor such as that disclosed in Japanese Patent Application Laid-Open Hei 9-26343, is well-known as a heated resistor type air flow sensor in which a thin-film sensor region is formed on a semiconductor (for example, silicone) substrate, and only one side of a sensor element 10 is attached to a cut-out space in a sensor-supporting part (a floating type support structure: a cantilever support structure) as shown in FIG. 19.
If the whole adhesion face of a sensor element is adhered to a sensor-supporting member, it is highly likely that the thin-film sensor region will be damaged in the adhesion process, and the adhesion part between the sensor element and the supporting part may also be damaged by a thermal cycle in some operation conditions, due to the difference between the linear expansion coefficients of the sensor element and the supporting member. Therefore, the above-mentioned floating type support structure is usually adopted.
However, in this floating type support structure, there is a problem that it is difficult to keep both the top surfaces of the sensor element and the sensor-supporting member in the same plane in the adhesion process. If a difference in level between the top surfaces of the sensor element and the sensor-supporting member is generated due to a mal-control for the thickness of the adhesive agent, and so on in the heated resistor type air flow sensor including a minute sensor element formed with a semiconductor processing technique, the flow pattern of air on the top surface of the sensor element will change, which in turn will cause a variation in characteristics of the heated resister type air flow sensor. Moreover, there is always a gap between the sensor element and the supporting member, and air flows onto the back surface of the sensor element through the gap. Thus, since a cavity space under the sensor region communicates with the gap in the floating type supporting structure, there is a problem that it is difficult to completely prevent the undesirable air flow into the cavity space. Further, since the spread-out of the adhesive agent onto the top surfaces of the sensor element and the supporting member cannot be controlled in the adhesion processing, the sensor element is highly likely to be damaged by the leakage of the adhesive agent into the cavity space. Furthermore, in the case when the adhesive agent is dropped onto the supporting member in order to adhere the sensor element to the supporting member, if air is mixed with the dropped adhesive agent in the adhesion processing, and the mixed air remains in the adhesive agent, this will cause a harmful effect upon the later-performed bonding process of the sensor element and an external signal processing circuit.
The present invention has been achieved with consideration to the above-described problems, and is aimed at solving those problems.
The above objective of the present invention is to achieve the provision of an air flow sensor comprising: a sensor element with a heated resistor formed on a semiconductor substrate via an electrical insulation layer; a sensor-supporting member containing a cut-out space in which the sensor element is fixed; a first region in which the sensor element contacts the sensor-supporting member in the cut-out space; and a second region in which the sensor element is adhered to the sensor-supporting member with adhesive agent put into grooves formed on the back face of the sensor element in the second region.
The grooves into which the adhesive agent is poured are shaped, for example, by an anisotropic etching method, and the generation of a difference in level between the top surfaces of the sensor element and the sensor-supporting member can be prevented by adhering the sensor element and the sensor-supporting member with the adhesive agent poured into the grooves. Further, by making the thickness of the adhesive agent between the sensor element and the sensor-supporting member zero, the communication of the gap between the sensor element and the sensor-supporting member and the cavity space is severed, which in turn can prevent the generation of an undesirable air flow into the cavity space. Furthermore, by letting an excess amount of the adhesive agent flow in a predetermined direction, its spread-out onto the top surface of the sensor element can be prevented.
In addition, by forming notches or grooves on the adhesion face of the sensor-supporting member, the above improvement can be achieved, and the remaining of air bubbles in the adhesion agent can also be prevented.